Carnegie Mellon University and Mspin
- Institution: Carnegie Mellon University
- Market: Academia – Department of Chemistry
- Problem: Three tetranortriterpenoids were isolated as hemiacetals (the C-23 epimeric xylorumphiins M− O (4−6 )), which could not be purified due to the equilibration of the hemiacetal epimers in solution, hence giving complex spectra. In order to enable purification, compounds were acetylated and the R and S epimers could be separated. However, for these C-23 epimers, use of standard NOESY and ROESY techniques did not permit their differentiation, due to long distances between H-23 and protons from the main skeleton.
The relative configuration of the new triterpenoids was determined by Dr. Gil and coworkers using the MSpin RDC module. 1DCH RDCs were collected for the C-23 epimers 5a and 5b on a compressed PMMA gel by using HSQC-F1 coupled experiments. The MSpin program was fed with the experimental RDCs and the computed structures for the possible C23 diastereoisomers and the relative configuration of both epimers of the molecule was established on the basis of MSpin RDC fittings. The RDC technique proved to be essential for the determination of configuration in the remote C23 side-chain center. Once the configuration at C-23 of each epimer (5a and 5b) was unambiguously determined by RDCs, the configuration of the related compounds (4a, 4b, 4c, 4d and 6a) was determined by direct comparison of 1H and 13C NMR chemical shifts.
Waratchareeyakul, W.; Hellemann, E.; Gil, R. R.; Chantrapromma, K.; Langat, M. K.; Mulholland, D. A. Application of Residual Dipolar Couplings and Selective Quantitative NOE to Establish the Structures of Tetranortriterpenoids From Xylocarpus rumphii. J. Nat. Prod. 2017, 80, 391–402.